Theory & Procedure, Characteristics of Flowers | Additional Study Material for NEET PDF Download

Our objective is to study the characteristics of the following plants- Petunia, Lathyrus, Asparagus and Onion.

Theory

Let's begin with the basic anatomy of flowers.

Flower is a reproductive organ of the angiosperm plant that contains thalamus and floral leaves. A typical flower consists of four types of floral leaves called sepals, petals, stamens and carpels in distinct whorls normally known as calyx, corolla, androecium and gynoecium respectively. A flower in which all the four whorls are present (Sepal, Petal, Carpels, Stamens) are called as complete.The stalk of a flower  is called as pedicel that holds the actual flower up in the air.

If a flower having both male and female sex organs are called bisexual and a flower having either only male or female organs is called unisexual. A flower having bract at its tip is called bracteate and without bracts is called ebracteate. A flower without a stalk or pedicel is called sessile and a flower having a stalk is called pedicellate. A flower may be trimerous, tetramerous or pentamerous when the floral leaves of each whorl are in multiple of 3, 4 or 5, respectively. 

Based on the symmetry of the flowers can be described in to following types:  

1. Actinomorphic: Flowers can be divided into two halves through any vertical plane.

2. Zygomorphic: Flowers can be divided into two equal halves only along one vertical plane.

3. Asymmetrical: Flowers which cannot be divided into equal halves by any plane.

Based on the position of calyx, corolla, and androecium with respect of ovary, the flowers are described as following:

1.Hypogynous (Superior ovary): Gynoecium occupies the highest position while the other parts are situated below it.

2.Perigynous (Half inferior): If gynoecium is situated in the centre and other parts of the flower are located on the rim of the thalamus almost at the same level.

3.Epigynous (Inferior ovary): The ovary situated in a flask shaped thalamus and other parts of flower arise above the ovary.

Parts of a flower

Each flower normally has four floral whorls, calyx, corolla, androecium and gynoecium.

1.Calyx

The calyx is the outermost whorl of the flower and is called sepals. Sepals looks like green leaves that are seen underneath the flower. Calyx may be gamosepalous (sepals united) or polysepalous (sepals free).

2.Corolla

The corolla is the second whorl of the flower and is composed of petals. Petals are the most colorful parts of a flower. Corolla may be gamopetalous (petals united) or polypetalous (petals free).

The arrangements of sepals or petals in the floral bud, with respect to the members of the same whorls are called aestivation.

1.Valvate: When sepals or petals do not overlap one another at the margin, without overlapping.

2.Twisted: Sepals or petals overlap with the next sepal or petal.

3. Imbricate: If the margins of sepals or petals overlap one another but not in any particular direction are called imbricate aestivation.

4.Vexillary: Among five petals, the largest petal (standard) overlaps the two lateral petals (wings) which in turn overlap two smallest anterior petals (Keel).

3. Androecium

Androecium is the third whorl of the flower which contains the male reproductive organ, stamens.  A stamen consists of an anther and a filament.

The stamens are variously fused among themselves. They can be of the following type:

1. Monoadelphous: stamens may be united into one bunch or one bundle. e.g., China rose
2. Diadelphous: stamens may be united into two bundles. e.g., pea
3. Polyadelphous: stamens may be united into more than two bundles. e.g.,  Citrus

Based on the attachment of filament to anther, it can be of the following type:

1. Basifixed: Filament of stamen is attached to base of the anther.
2. Adnate: Filament attached along the whole length of anther.
3. Dorsifixed: Filaments attached to the back of anther.
4. Versatile: Anther lobes attached with filament in the middle portion with both ends free

4.Gynoecium

Gynoecium is the innermost whorl of a flower, consisting of one or more units called carpels. Each carpel includes an ovary, a  style and a stigma.

Carpels are of two types depending upon fusion:

1. Apocarpous: Carpels free from each other e.g.,  lotus
2. Syncarpous: Carpels fused with each other e.g., mustard.

The arrangement of placentae bearing ovules inside the ovary is called placentation. It is of following types.

1. Marginal: The placenta forms a ridge along the ventral suture of the ovary and the ovules are borne on this ridge e.g., pea.
2. Axile: Margins of carpels fuse to form central axis. e.g., tomato
3. Parietal: The ovules develop on the inner wall of the ovary. e.g., mustard.
4. Free central: Ovules borne from central axis and lacking septa. e.g., chilly.
5. Basal: Placenta develops at the base of the ovary. e.g., sunflower.

We will now look at the characteristics of some plants based on their family.

1 . Petunia Alba (Family: Solanaceae)

Floral Characteristics:

• Flower:Ebracteate, pedicellate, complete, actinomorphic, bisexual, pentamerous, hypogynous.
• Calyx:Sepals 5, Gamosepalous, green, valvate aestivation.
• Corolla:Petals 5, Gamopetalous, white/purple, valvate aestivation.
• Androecium:Stamens 5, Epipetalous, Anther basifixed, Dithecous.
• Gynoecium:Bicarpellary, syncarpous, ovary superior, axile placentation, swollen placenta.

Reason for identification:

Persistent sepals, infundibuliform corolla, stamen epipetalous, ovary obliquely placed, axile placentation with swollen placenta.

2. Sweet Pea [Family: Fabaceae]

Floral Characteristics:

• Flower:Bracteate, pedicellate, complete, zygomorphic, bisexual, pentamerous.
• Calyx: Sepals 5, Gamosepalous, green, valvate aestivation, odd sepal anterior.
• Corolla: Petals5, polypetalous, papilionaceous, vexillary aestivation.
• Androecium:Stamens 10, diadelphous 9 +1, 9 stamens united at base and 10th stamen is free, anther basifixed, dithecous.
• Gynoecium: Monocarpellary, ovary superior, unilocular, marginal placentation.

Reason for identification:

Flowers zygomorphic with papilionaceous corolla, stamens diadelphous, monocarpellary gynoecium, ovary unilocular with marginal placentation.

3. Asparagus [Family: Liliaceae]

Floral Characteristics:

• Flower: Bracteate, pedicellate, incomplete, actinomorphic, bisexual, hypogynous, trimerous
• Tepals: Tepals 6, two whorls of three each, white.
• Androecium: Stamens 6, antiphyllous, anther basifixed, Dithecous.
• Gynoecium: Tricarpellary, syncarpous, ovary superior, trilocular, axile placentation.

Reason for identification:

Flowers are trimerous, petaloid perianth in two whorls of 3 each, stamens 6 in two whorls of 3 each, epiphyllous, ovary tricarpellary, trilocular with axile placentation.

4. Onion [Family: Liliaceae]

Floral Characteristics:

• Flower: Bracteate, pedicellate, incomplete, actinomorphic, bisexual, hypogynous, trimerous
• Tepals: Tepals 6, two whorls of three each, petaloid, white.
• Androecium: Stamens 6, polyandrous, antiphyllous, anther basifixed, dithecous.
• Gynoecium: Tricarpellary, syncarpous, ovary superior, trilocular, axile placentation.

Reason for identification:

Flowers are trimerous, petaloid perianth in two whorls of 3 each, stamens 6 in two whorls of 3 each, epiphyllous, ovary tricarpellary, trilocular with axile placentation.

Materials Required

Real Lab Procedure

• Take one flower and make the following observations.
• Note down whether it has a pedicel, bracts, epicalyx or not.
• Identify the presence of different whorls of floral leaves such petals, sepals, androecium and gynoecium.
• Carefully remove the floral leaves of each whorl and place them whorlwise on a white paper, i.e., sepals, petals, stamens and carpels
• Count the number of floral leaves of each whorl.
• Using a blade, cut thin section of ovary. Mount the section on a slide in a drop of water and examine under dissecting microscope. 
• Count the number of chambers in anther and ovary and number of ovules in each chamber locule.
• Find the nature of ovary, style stigma and placentation.

Simulator Procedure (as performed through the Online Labs)

• To select a sample of a flower, click on the corresponding icon.
• Click on the “Cross-sectional view”button to view the cross section of the sample.
• To count the number of floral leaves of each whorl of the selected sample, click on the button, “Count the florals”.
• Drag the forceps towards the flower and remove the floral leaves of each whorl and drag to place them on the right pane.
• To repeat the experiment, click on the ‘Reset’ button.